Understanding Respiration: IGCSE Biology Explained

Respiration is not breathing

This is the single most important sentence in the entire topic, so it deserves its own opening line. Respiration is a chemical process that happens inside cells. Breathing is the physical movement of air in and out of the lungs. They are connected, of course, because breathing delivers the oxygen that cells need for aerobic respiration, but they are not the same thing. Confusing the two is one of the most common reasons IGCSE Biology students lose marks, and examiners have been penalising it for decades.

Respiration happens in every living cell, in every living organism, all the time. Plants respire. Bacteria respire. Your muscle cells respire while you sleep. It is the universal energy-releasing process of life, and your Cambridge exam will test you on it from several angles.

What the energy is used for

Before looking at the chemistry, it helps to understand why cells need this energy in the first place. The IGCSE syllabus lists seven specific uses:

• Muscle contraction - every movement your body makes, from blinking to sprinting
• Protein synthesis - building new proteins from amino acids at ribosomes
• Cell division - the energy-intensive process of copying DNA and splitting into new cells
• Active transport - moving substances against a concentration gradient across cell membranes
• Growth - producing new cells and enlarging existing ones
• Nerve impulses - transmitting electrical signals along neurones
• Maintaining a constant body temperature - mammals and birds use energy from respiration to generate heat

Aerobic respiration

Aerobic respiration is the form that uses oxygen. It is the main energy-releasing pathway in most organisms most of the time, and it extracts far more energy per molecule of glucose than any alternative.

The equations

Word equation (Core):

glucose + oxygen -> carbon dioxide + water

Notice that the equation is the exact reverse of photosynthesis. The same molecules appear on both sides, just swapped around. Photosynthesis stores energy in glucose; respiration releases it. Examiners like to test whether you understand this symmetry.

Where it happens

Aerobic respiration takes place in the mitochondria of cells. This is worth remembering because exam questions about cell structure sometimes ask which organelle is responsible for energy release. Cells that need a lot of energy, such as muscle cells and liver cells, contain large numbers of mitochondria.

Anaerobic respiration

Anaerobic respiration is the release of energy from glucose without oxygen. It is less efficient than aerobic respiration, producing much less energy per glucose molecule, but it allows cells to continue working when oxygen supply cannot keep up with demand.

In animals (including humans)

Word equation:

glucose -> lactic acid

When you sprint flat out, your muscles demand energy faster than your lungs and blood can deliver oxygen. Your muscle cells switch to anaerobic respiration as a short-term backup. Lactic acid builds up in the muscles, causing the burning sensation and fatigue you feel during intense exercise.

In yeast (and some plants)

Word equation:

glucose -> ethanol + carbon dioxide

This process is called fermentation. It is the basis of bread-making (the carbon dioxide makes dough rise) and biofuel production (the ethanol is collected as fuel). Unlike animal anaerobic respiration, fermentation produces two products rather than one.

Comparing the two types

Oxygen debt

After intense exercise, you keep breathing heavily even after you have stopped running. This is not habit. Your body is repaying an oxygen debt.

During the exercise, your muscles produced lactic acid through anaerobic respiration. That lactic acid needs to be broken down, and breaking it down requires oxygen. The extra oxygen you breathe in after exercise is used to oxidise the lactic acid, converting it into carbon dioxide and water (or, in the liver, converting it back to glucose). The total amount of extra oxygen needed to clear the lactic acid is called the oxygen debt.

Think of it like a credit card. You "borrowed" energy without oxygen during the sprint. Now you have to "pay back" by taking in the oxygen you skipped. The heavier you breathe after exercise, the larger your oxygen debt was.

Investigating respiration: the yeast experiment

A classic IGCSE practical involves investigating how temperature affects the rate of respiration in yeast. The setup is straightforward:

1. Mix yeast with glucose solution in a test tube or flask.
2. Attach a delivery tube leading to a second container of limewater (or collect gas in a gas syringe).
3. Place the flask in a water bath set to a specific temperature.
4. Count the number of carbon dioxide bubbles produced per minute, or measure the volume of gas collected over a fixed time.
5. Repeat at different temperatures (e.g. 20, 30, 40, 50, 60 degrees Celsius).

Expected results

As temperature increases from room temperature, the rate of respiration rises because the enzymes involved work faster (more kinetic energy means more frequent enzyme-substrate collisions). The rate peaks at around 35-40 degrees Celsius, which is the optimum temperature for yeast enzymes. Above this point the rate drops sharply because the enzymes become denatured: their active sites change shape permanently and can no longer catalyse the reactions.

Effects of exercise on breathing

Physical activity increases both the rate (number of breaths per minute) and the depth (volume of air per breath) of breathing. The reason is simple: working muscles demand more oxygen for aerobic respiration and produce more carbon dioxide as a waste product. Faster, deeper breathing brings more oxygen into the lungs and removes carbon dioxide more quickly.

The heart rate also increases during exercise. A faster heartbeat pumps oxygenated blood to the muscles more quickly and carries carbon dioxide away to the lungs for removal. Together, the increased breathing rate and increased heart rate ensure that the muscles receive the oxygen they need and that waste products do not accumulate to harmful levels.

Examiners sometimes present data tables showing breathing rate and depth at rest versus during exercise and ask you to describe and explain the trend. Always link your explanation back to increased respiration in muscles, greater oxygen demand, and greater carbon dioxide production. If the data also shows heart rate, mention the connection between faster circulation and faster delivery of oxygen to respiring cells.

Common exam mistakes

1. Saying respiration is breathing. Respiration is a chemical reaction inside cells. Breathing is the mechanical ventilation of the lungs. They are linked but not the same. Writing "respiration is when you breathe in and out" will lose you marks immediately.
2. Forgetting to specify "anaerobic" when describing fermentation. If yeast is respiring without oxygen, you must state that the process is anaerobic. Leaving out this word costs a mark.
3. Claiming anaerobic respiration produces "no energy." It does produce energy, just much less than aerobic respiration. The correct comparison is "less energy" or "a smaller amount of energy," not none.
4. Mixing up the products. In animals, anaerobic respiration produces lactic acid (not ethanol). In yeast, it produces ethanol and carbon dioxide (not lactic acid). Swapping these is a common and costly error.
5. Writing that enzymes are "killed" at high temperatures. Use the word denatured. Enzymes are proteins, not living things, so they cannot be killed.
6. Confusing respiration with photosynthesis. Respiration releases energy by breaking down glucose. Photosynthesis stores energy by building glucose. They use the same molecules but in opposite directions. Both happen in plant cells, but respiration occurs all the time while photosynthesis only occurs in the light.

Self-check questions

1. State three uses of energy in living organisms.
2. Write the word equation for aerobic respiration.
3. Write the word equations for anaerobic respiration in (a) animals and (b) yeast.
4. Explain why aerobic respiration releases more energy per glucose molecule than anaerobic respiration. (Extended)
5. Describe what is meant by oxygen debt and explain why breathing rate remains high after exercise stops.
6. In the yeast experiment, explain why the rate of carbon dioxide production decreases above 40 degrees Celsius.
7. A student writes: "Plants do not respire because they photosynthesise." Explain why this statement is incorrect.